Method of making updoped cladding by using silicon tertrachloride as the dopant

ABSTRACT

One embodiment of the disclosure relates to a method of making an optical fiber comprising the steps of: (i) exposing a silica based preform with at least one porous glass region having soot density of ρ to a gas mixture comprising SiCl 4  having SiCl 4  mole fraction y SiCl4  at a doping temperature T dop  such that parameter X is larger than 0.03 to form the chlorine treated preform, wherein 
             X   =     1     1   +     [       (     ρ       ρ   s     -   ρ       )     ⁢       0.209748   ⁢           ⁢     T   dop     ⁢     Exp   ⁡     [       -   5435.33     /     T   dop       ]           y     SiCl   ⁢           ⁢   4       3   /   4           ]               
and ρ s  is the density of the fully densified soot layer; and (ii) exposing the chlorine treated preform to temperatures above 1400° C. to completely sinter the preform to produce sintered optical fiber preform with a chlorine doped region; and (iii) drawing an optical fiber from the sintered optical preform.

This application is a continuation of U.S. application Ser. No.14/467,369 filed on Aug. 25, 2014 which claims the benefit of priorityto U.S. application Ser. No. 61/874,555 filed on Sep. 6, 2013, thecontents of which are relied upon and incorporated herein by referencein its entirety.

BACKGROUND

The present invention relates to optical fibers, and more specificallyto optical fibers having a cladding updoped by silicon tertrachloride.

No admission is made that any reference cited herein constitutes priorart. Applicant expressly reserves the right to challenge the accuracyand pertinency of any cited documents.

SUMMARY

One embodiment of the disclosure relates to a method of making anoptical fiber preform comprising the steps of:

-   -   (i) exposing a silica based preform with at least one porous        glass region having soot density of ρ to a gas mixture        comprising SiCl₄ having SiCl₄ mole fraction y_(SiCl4) (for        example, of less than 0.03) at a doping temperature T_(dop) such        that parameter X is larger than 0.03 to form the chlorine        treated preform, wherein

$X = \frac{1}{1 + \left\lbrack {\left( \frac{\rho}{\rho_{s} - \rho} \right)\frac{0.209748\mspace{11mu} T_{dop}{{Exp}\left\lbrack {{- 5435.33}/T_{dop}} \right\rbrack}}{y_{{SiCl}\; 4}^{3/4}}} \right\rbrack}$and ρ_(s) is the density of the fully densified soot layer and ρ<ρ_(s);and

-   -   (ii) exposing the chlorine treated preform to temperatures above        1400° C. to completely sinter the preform to produce sintered        optical fiber preform with a chlorine doped region having a        chlorine doped. In the embodiments disclosed herein ρ<0.8        gm/cm³, preferably ρ<0.6 gm/cm³, for example ρ<0.6 gm/cm³, and        in some embodiments ρ<0.5 gm/cm³.

According to some embodiments 0.005<y_(SiCl4)<0.03. According to someembodiments X>0.04. According to some embodiments 0.3 gm/cm³<ρ<1 gm/cm³and 2.1 gm/cm³<ρ_(s)<2.3 gm/cm³, for example ρ_(s)=2.2 gm/cm³. In someembodiments 0.3 gm/cm³<ρ<0.8 gm/cm³, for example 0.35 gm/cm³<ρ<0.6gm/cm³ and 2.15 gm/cm³<ρ_(s)<2.25 gm/cm³. In some embodiments T_(dop) isless than 1225° C. In some embodiments 1000° C.<T_(dop)<1200° C., and insome embodiments T_(dop)<1175° C.

According to some embodiments the chlorine doping profile of thechlorine doped region is such that the ratio of the concentration ofdoped chlorine in the inner portion of the Cl doped region toconcentration of doped chlorine in the outer portion of the Cl dopedregion is >0.75. According to some embodiments the mole fraction ofSiCl₄ in the gas mixture y_(SiCl4) is larger than 0.005. According tosome embodiments the mole fraction of SiCl₄ in the gas mixture y_(SiCl4)is larger than 0.015.

According to some embodiments a method of making an optical fiberpreform comprises the steps of:

-   -   (i) exposing a silica based preform with at least one porous        glass region having soot density of ρ to a gas mixture        comprising SiCl₄ having SiCl₄ mole fraction y_(SiCl4) at a        doping temperature T_(dop) such that parameter X is larger than        0.03 to form a chlorine treated preform, wherein

$X = \frac{1}{1 + \left\lbrack {\left( \frac{\rho}{\rho_{s} - \rho} \right)\frac{0.21T_{dop}{{Exp}\left\lbrack {{- 5435.33}/T_{dop}} \right\rbrack}}{y_{{SiCl}\; 4}^{3/4}}} \right\rbrack}$and ρ_(s) is the density of the fully densified soot layer, and theinside of the soot layer (e.g., at least a 1 mm wide inner most portionof this layer during this SiCl₄ exposure step) is at a temperatureT_(inside) that is at least 25° C. higher than the doping temperatureT_(dop)(ii) exposing the chlorine treated preform to temperatures above1400° C. to completely sinter the preform to produce sintered opticalfiber preform with a chlorine doped region having a chlorine dopedprofile.

According to some embodiments a method of making an optical fiberpreform comprises the steps of:

-   -   (i) exposing a silica based preform with at least one porous        glass region having soot density of ρ to a gas mixture        comprising SiCl₄ at a doping temperature T_(dop) such that SiCl₄        mole fraction y_(SiCl4) at the end of the SiCl₄ exposure step is        at least 1.5 times the SiCl₄ mole fraction at the beginning of        the SiCl₄ exposure step; and    -   (ii) wherein exposing of the silica based preform with at least        one porous glass region having soot density of ρ to a gas        mixture comprising SiCl₄ having SiCl₄ mole fraction y_(SiCl4) is        performed at a doping temperature T_(dop) such that parameter X        is larger than 0.03 to form the chlorine treated preform,        wherein

$X = \frac{1}{1 + \left\lbrack {\left( \frac{\rho}{\rho_{s} - \rho} \right)\frac{0.21T_{dop}{{Exp}\left\lbrack {{- 5435.33}/T_{dop}} \right\rbrack}}{y_{{SiCl}\; 4}^{3/4}}} \right\rbrack}$and ρ_(s) is the density of the fully densified soot layer, and theinside of the soot layer (for example, at least a 1 mm wide inner mostportion of this layer during this exposure step) is at a temperatureT_(inside) that is at least 25° C. higher than the doping temperatureT_(dop)

-   -   (iii) exposing the chlorine treated preform to temperatures        above 1400° C. to completely sinter the preform to produce        sintered optical fiber preform with a chlorine doped region        having a chlorine doped profile.

According to some embodiments the method of making an optical fiberpreform comprises the steps of:

-   -   (ii) exposing a silica based preform with at least one porous        glass region having soot density of ρ to a gas mixture        comprising SiCl₄ at a doping temperature T_(dop) such that SiCl₄        mole fraction y_(SiCl4) at the end of the SiCl₄ exposure step is        at least 1.5 times the SiCl₄ mole fraction at the beginning of        the SiCl₄ exposure step; and    -   (ii) exposing the chlorine treated preform to temperatures above        1400° C. to completely sinter the preform to produce sintered        optical fiber preform with a chlorine doped region having a        chlorine doped profile.

According to some embodiments the method of making an optical fiberpreform comprises the steps of:

-   -   (i) exposing a silica based preform with at least one porous        glass region having soot density of ρ<0.8 gm/cm³ to a gas        mixture comprising SiCl₄ having SiCl₄ mole fraction y_(SiCl4) of        less than 0.03 at a doping temperature T_(dop)<1225° C.; and    -   (ii) exposing the chlorine treated preform to temperatures above        1400° C. to completely sinter the preform to produce sintered        optical fiber preform with a chlorine doped region having a        chlorine doped profile.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and operationof the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates refractive index profile of an embodiment of anoptical fiber as disclosed herein;

FIG. 2 is a plot of chlorine concentration of the optical preform layermanufactured by exposing a silica soot layer to a gas mixture havingSiCl₄ mole fraction of 0.026 at a doping temperature of 1300° C.;

FIG. 3 is a plot of chlorine concentration of the optical preform layermanufactured by exposing a silica soot layer to a gas mixture havingSiCl₄ mole fraction of 0.02 at a doping temperature of 1250° C.;

FIG. 4 is a plot of chlorine concentration of the optical preform layermanufactured by exposing a silica soot layer to a gas mixture havingSiCl₄ mole fraction of 0.02 at a doping temperature of 1250° C.; and

FIG. 5 illustrates uniform chlorine concentration in Cl doped outerlayers of three embodiments of optical fiber preforms doped according toone embodiment of the present invention.

DETAILED DESCRIPTION

Additional features and advantages will be set forth in the detaileddescription which follows and will be apparent to those skilled in theart from the description or recognized by practicing as described in thefollowing description together with the claims and appended drawings.

Various embodiments will be further clarified by the followingexample(s).

One exemplary fiber embodiment 10 is shown in FIG. 1, includes a centralglass core region 1 comprising maximum refractive index delta percentΔ₁. A first inner cladding region—i.e., the depressed inner claddingregion 2 surrounds central core region 1, the first inner claddingregion 2 (also referred to as a trench herein) comprising refractiveindex delta percent Δ₂. Outer cladding region 3 surrounds first innercladding region 2 and comprises Δ₃. In preferred embodiments, Δ₁>Δ₃>Δ₂.In the embodiment illustrated in FIG. 1, regions 1, 2, 3 are immediatelyadjacent to one another. However, this is not required, andalternatively additional core or cladding regions may be employed. Forexample, another inner cladding region (not shown) may be employed whichis situated between the core region 1 and inner cladding region 2.

Central core region 1 comprises an outer radius r₁ which is defined aswhere a tangent line drawn through maximum slope of the refractive indexof central core region 1 crosses the zero delta line. Core region 1preferably exhibits a refractive index delta percent, Δ₁(%), betweenabout 0.3 to 0.7 (relative to pure silica), and in some embodimentsbetween about 0.3 to 0.5. In some embodiments, Δ₁ (%) is preferablybetween 0.38 and 0.42. Core radius r₁ is preferably between 3 and 10microns, more preferably between about 4.0 to 7.0 microns. Central coreregion 1 may comprise a single segment, step index profile.Alternatively, in some embodiments, central core region 1 exhibits analpha greater than 0.5 and less than 10, and in some embodiments lessthan 7.5, less than 6, or less than 4. In some preferred embodiments,central core region 1 exhibits an alpha less than 7.5, less than 5, orless than.

In the embodiment illustrated in FIG. 1, inner cladding region 2surrounds central core region 1 and comprises inner radius r₁ and outerradius r₂, where r₁ being defined as above and r₂ being defined as wherethe refractive index profile curve crosses the zero delta line. In somecases the refractive index in region 2 is essentially flat. In someembodiments, the first inner cladding region contains fluorine. In someembodiments, the first inner cladding region contains less than 0.02 wt% fluorine. In some embodiments, the inner cladding region 2 is puresilica. The inner cladding region 2 preferably exhibits a width betweenabout 3 to 13 microns, more preferably 4 to 12 microns, even morepreferably between about 7 to 9 microns. In some embodiments, R₂ isgreater than 9 microns, or greater than 10 microns and less than 15microns.

Outer cladding region 3 surrounds the depressed annular region 2 andcomprises refractive index delta percent Δ₃ which is higher than theindex Δ₂ of inner cladding region 2, thereby forming a region which isan “updoped” outer cladding region 3 with respect to inner claddingregion 2, by adding chlorine sufficient to increase the refractive indexof the outer cladding region. Thus, outer cladding region 3 comprises ahigher refractive index than inner cladding region 2, and preferablycomprises refractive index delta percent Δ₃ which is greater than0.005%, preferably at least 0.001%, for example at least 0.015%, 0.02%,0.03%, 0.05%.

In some fiber embodiments, the third annular region (also referred toherein as outer cladding layer or outer cladding region 3) compriseschlorine (Cl) in an amount greater than 1000 ppm or more, and in someembodiments preferably greater than 1500 ppm, and, in some embodiments,most preferably greater than 2000 ppm (0.2%) by weight (e.g., 2200 ppm,2500 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, 10000 ppm, or therebetween). Chlorine concentration is described herein in units of partsper million by weight (abbreviated as ppm wt. or ppm, herein). It hasbeen determined that 10000 ppm (1 wt %) chlorine increases the index ofthe outer cladding by about 0.1% delta.

The optical fibers 10 disclosed herein may be drawn from consolidatedoptical fiber preforms 100 using known fiber draw methods and apparatus.The preforms 100 are made by: (i) creating a silica soot preform 100Acomprising of core region 1 and inner clad region 2, sintering it toglass and producing a preform 100C comprising the core region 1 (whichcorresponds to the core region 1 of the fiber) and the first innercladding region 2 (trench region, which corresponds to the innercladding region 2 of the fiber); (iii) overcladding the resultantsintered preform 100C with silica soot layer, then exposing silica sootlayer overclad to a chlorine dopant precursor to dope it with chlorine(e.g., SiCl₄ exposure step) to form preform 100D with chlorine dopedouter soot layer, and finally sintering the chlorine doped soot layer toglass to create the final preform 100 with an outer cladding region 3.The soot layer is formed by the deposition of pyrogenically generatedsilica soot particles. According the following embodiments, the sootlayers described herein have a deposit density of less than 0.8 gm/cm³,more preferably less than 0.7 gm/cm³, and even more preferably less than0.6 gm/cm³.

More specifically, silica soot can be made by flame hydrolysis or flamecombustion process known to one skilled in the art. To produce puresilica soot, high purity silicon containing precursor compounds (e.g.,silicon tetrachloride, octamethylcyclotetrasiloxane, etc.) areintroduced into a flame (e.g., the flame of the burner), for example anoxygen-hydrocarbon, or oxygen-hydrogen flame to form “seeds” of solidsilicon dioxide, called silica soot. Silica soot is then deposited ontoa bait rod or on layer glass (either porous or completely sintered) toform a porous soot layer (porous silica glass layer) with the porosityof less than of less than 0.8 gm/cm³, more preferably less than 0.7gm/cm³, and even more preferably less than 0.6 gm/cm³. Typically theaverage size of silica soot particles is in a submicron range with sootparticle sizes ranging between 10 nm and 0.5 um. An non exhaustive listof examples of diameters of soot particles utilized here in are 0.01 μm,0.05 um, 0.075 μm, 0.1 μm, and 0.2 μm. Typical soot particles (silicabased soot) have a surface areas of more than about 5 m²/g (meter² pergram of sample) or higher and less than 50 m²/g, for example 10 m²/g, 20m²/g, 30 m²/g or 40 m²/g. That is, silica soot is sub-micron particlesof silica generated by in the flame by oxidation or/or hydrolysis.Silica based soot may contain other dopants, for example if anotherprecursor such as Ge containing precursor or another dopant is alsoprovided to the oxygen fed flame. The manufacture of cylindrical bodiesof synthetic vitreous silica is a well-known process of great importancein the production of optical fibers. One of the most widely usedtechniques is the so-called Outside Vapor Deposition (OVD) process, inwhich silica soot is generated in the flame of one or more burners fedwith a chosen silica precursor, usually in the form of vapor butsometimes in the form of an aerosol spray. The precursor is converted inthe flame by oxidation or hydrolysis into a stream of nanoparticles ofsilica (silica soot), and this stream is directed at a rotatingsubstrate or mandrel on which the particles collect as a porous silicasoot body (soot layer).

The optical fiber preform's inner cladding region 2 may optionally bedoped with fluorine. That is, the preform layers corresponding to thefiber's core and the trench region of the cladding are deposited as asoot to create a soot preform 100A (for example by the OVD process), andthe optional fluorine doping of at least the preform portioncorresponding to the trench region 2 is performed in a single step fromoutside, preferably at temperatures at below 1300° C., more preferablybelow 1290° C., with fluorine precursor (for example with SiF₄, CF₄,C₂F₆, and/or SF₆) to produce a preform 100B with a F-doped region.(Alternatively, as described above, the inner cladding region 2 of thepreform can be made of essentially pure silica—in this case fluorinedoping step is unnecessary.) The preform 100B is then consolidated tofull densification to form preform 100C, overclad with silica sootlayer, and then exposed to a chlorine dopant precursor (e.g., SiCl₄exposure) to dope the silica soot layer overclad with chlorine (thusforming preform 100D with Cl doped outer soot layer) and, finally,sintering the chlorine doped soot layer to glass to create the finalpreform 100 with an outer cladding region 3. In the embodimentsdisclosed herein the chlorine doping of the outer cladding region 3 isachieved by exposing an optical preform with at least one soot layer toa chlorine dopant during the pre-heat and prior to the sintering part ofthe consolidation process.

More specifically, according to at least some embodiments the outercladding layer 3 is created to by depositing silica soot onto theoptical fiber preform 100C or by or by placing a silica soot sleevearound the optical fiber preform 100C, and doping of optical fiberpreform's region 3 with SiCl₄ as the chlorine dopant at temperaturesbetween 1000° C. and 1250° C. (e.g., 1025° C. 1225° C.), preferably1050° C.-1175° C. to make the optical fiber preform 100D, which is thenconsolidated to a final optical fiber preform 100. The final preform 100is utilized to draw optical fiber 10 for use in telecommunication orother systems.

It has been determined that in order to get good quality glass that isessentially free of seeds, haze and defects, in one exemplaryembodiment, the process for chlorine doping of the preform's region 3comprises: (i) exposure of the soot layer to SiCl₄ at furnacetemperatures of less than 1225° C. and preferably less than 1200° C.(temperature at which no significant densification takes place), (ii)followed by sintering/consolidation of the SiCl₄ exposed soot later attemperatures above 1400° C. in a gas environment having SiCl₄ molefraction that is between 0 to 0.005. That is, preferably chlorinedoping/exposure with SiCl₄ is done at temperatures below 1220° C. andmore preferably below 1200° C. during the doping step, with thesintering of the Cl doped soot layer performed in an environment that isessentially free of SiCl₄ at temperatures above 1400° C., whichsignificantly improves the quality of the resultant preform.

In some embodiments, small amount of SiCl₄ may be added in the sinteringstep to suppress the loss of doped chlorine at high temperaturesencountered during sintering (>1400° C.), with the SiCl₄ concentrationin the sintering step is less than 0.25 of the SiCl₄ concentration inthe doping step, more preferably less than 0.2 of the SiCl₄concentration in the doping step, and even more preferably less than 0.1of the SiCl₄ concentration in the doping step.

The chlorine doping of the outer cladding region 3 may also be achievedby exposing a soot preform to a chlorine dopant during the sinteringpart of the consolidation process. One of the dopants that is veryefficient in doping chlorine is silicon tertrachloride (SiCl₄). However,because of the high reactivity of SiCl₄ as a dopant, diffusionallimitations can set in resulting in non-unifomities in radial and axialchlorine doping levels, particularly when chlorine doping soot layerswith thickness greater than 6 cm that are used in low cost, large scalemanufacturing. The non-uniformities in radial and axial chlorine dopinglevels are undesirable as these negatively impact optical fiber productperformance and attributes. Applicants discovered combinations of dopingtemperatures and SiCl₄ concentrations that surprisingly result inuniform axial and radial chlorine doping profiles in soot layers whichare part of large optical fiber preform.

For example, in some embodiments, to achieve the doping levels in theouter cladding layer 3 to produce the exemplary optical fiber indexprofiles described above, we determined that it is preferable to havethe SiCl₄ dopant mole fraction of greater than 0.001 but less than ofless than 0.03 in the gas mixture that is used to dope porous silicasoot layer with chlorine. Furthermore, we realized that because of theefficient reaction of SiCl₄ with silica to chlorine dope the silica sootlayer, the effective diffusivity of the SiCl₄ dopant is significantlyreduced from its pore diffusivity, where pore diffusivity is defined asthe SiCl₄ diffusivity in the pores of soot layer in absence of anyreaction. The ratio of the SiCl₄ effective diffusivity to porediffusivity is described by parameter X, wherein parameter X is givenas:

$X = {\frac{D_{eff}}{D_{pore}} = \frac{1}{1 + \left\lbrack {\left( \frac{\rho}{\rho_{s} - \rho} \right)\frac{0.21T_{dop}{{Exp}\left\lbrack {{- 5435.33}/T_{dop}} \right\rbrack}}{y_{{SiCl}\; 4}^{3/4}}} \right\rbrack}}$where D_(eff) is the effective diffusivity of the SiCl₄ through thepores of the soot preform and includes the impact of chlorine dopingreaction on diffusivity, D_(pore) is the pore diffusivity of the SiCl₄through the pores of the soot preform in absence of any reaction, ρ isthe average density of the soot layer (g/cm³), ρ_(s) is the density offully densified soot layer (˜2.2 g/cm³ for the case of silica soot),T_(dop) is the doping temperature (furnace temperature) at which dopingwith SiCl₄ is performed and y_(SiCl4) is the mole fraction of the SiCl₄in the gas mixture that is used for doping. The radial uniformity of thedoped chlorine level is strongly influenced by the effective diffusivityof SiCl₄ and therefore by parameter X. For the case when SiCl₄ effectivediffusivity is small, the diffusional limitations are present thatresult in higher amount of SiCl₄ doping on the outside portion (i.e.,the outmost portion) of the soot layer that is being doped compared tothe inner location (e.g., inner most portion) in the soot layer. Forexample, FIG. 2 illustrates chlorine profiles resulted when silica sootlayer of an optical fiber preform was exposed to a gas mixture havingSiCl₄ mole fraction of 0.026 at the doping temperature of 1300° C.(measured in gases directly adjacent to the preform), followed bycomplete consolidation/sintering in SiCl₄ free environment at atemperature above 1450° C. (Normalized position of 0 corresponds to theinner most location of the chlorine doped layer (i.e., the inner mostportion of the chlorine doped layer)). As can be seen, there is asignificant non-uniformity in this doped chlorine profile, whichnegatively impact product attributes. Conversely, higher effectivediffusivity (and therefore larger value of X) results in greaterpenetration of the SiCl₄ dopant through the porous soot preformresulting in more uniform doping levels. FIG. 3 illustrates chlorineprofile that resulted when silica soot layer of an optical fiber preformis exposed to a gas mixture having SiCl₄ mole fraction of 0.02 at thedoping temperature of 1250° C. followed by completeconsolidation/sintering in SiCl₄ free environment at a temperature above1450° C. FIG. 3 shows that non-uniform chlorine doped behavior in termsof radial doped profile is also observed for the doping temperature of1250° C. and SiCl₄ mole fraction of 0.02. In comparison, chlorine dopedprofiles that are substantially more uniform were obtained when dopingthe silica soot layer with of SiCl₄ mole fraction of 0.02 at 1125° C.(see, for example, FIG. 4).

We discovered that good radial and axial uniformity of Cl dopingconcentration is achieved for the combinations of the parameters in Eq[1] (soot layer density, doping temperature; and SiCl₄ mole fraction inthe gas mixture used for chlorine doping the soot layer) that result inparameter X having values larger than 0.029, preferably 0.03 or larger,more preferably larger than 0.035 and even more preferably larger than0.04. Preferably, the average soot density of the soot layer that isbeing treated with SiCl₄ is between 0.3 gm/cm³ and 1 gm/cm³, morepreferably between 0.4 gm/cm³ and 0.8 gm/cm³, and even more preferablybetween 0.4 gm/cm³ and 0.7 gm/cm³. In some embodiments, for the chlorinedoping levels needed in outer cladding region 3 of the profile shown inFIG. 1, the mole fraction of SiCl₄ in the gas mixture for chlorinedoping is preferably less than 0.03 and even more preferably less than0.0275. In these embodiments, the mole fraction of SiCl₄ in the gasmixture to be used for chlorine doping is preferably larger than 0.005,more preferably larger than 0.01 and even more preferably larger than0.015. In some embodiments the doping temperature is less than 1200° C.,more preferably less than 1175° C. and even more preferably less than1150° C. but greater than 1000° C. (The doping temperature furnacetemperature or gas temperature right next to the preform). The exposuretime of the soot layer to the gas mixture comprising the SiCl₄ dopant ispreferably greater than 30 minutes, more preferably greater than 60minutes and even more preferably greater than 90 minutes (but forexample, less than 10 hrs). Tables 1A to 1D show exemplary values ofparameter X for different combinations of doping temperatures T=T_(dop),and SiCl₄ mole fractions for soot layer density is 0.44 gm/cm³. Thebolded values represent the preferable range of X values (i.e., X valuesof 0.03 and higher). Applicants discovered that when X>0.03non-uniformities in radial and axial chlorine doping levels are reduced.Applicants discovered that when X>0.05 non-uniformities in radial andaxial chlorine doping levels are even further reduced. It is observedthat for values of SiCl₄ mole fraction, y_(SiCl4)>0.03, the values ofparameter X is generally higher than 0.03 for most doping temperaturesof interest, which can results in a non-uniformity.

TABLE 1A SiCl₄ Mole X at X at X at X at X at X at Fraction T = 1000° C.T = 1025° C. T = 1050° C. T = 1075° C. T = 1100° C. T = 1125° C. 0.0030.014 0.012 0.011 0.010 0.009 0.008 0.005 0.020 0.018 0.016 0.015 0.0130.012 0.008 0.027 0.024 0.022 0.020 0.018 0.017 0.010 0.033 0.030 0.0270.025 0.022 0.021 0.013 0.039 0.035 0.032 0.029 0.026 0.024 0.015 0.0440.040 0.036 0.033 0.030 0.028 0.018 0.049 0.044 0.040 0.037 0.034 0.0310.020 0.054 0.049 0.045 0.041 0.037 0.034 0.023 0.059 0.053 0.048 0.0440.041 0.037 0.025 0.063 0.057 0.052 0.048 0.044 0.040 0.028 0.067 0.0610.056 0.051 0.047 0.043 0.030 0.072 0.065 0.059 0.054 0.050 0.046

TABLE 1B SiCl₄ X at X at X at X at X at Mole T = T = T = T = T =Fraction 1150° C. 1175° C. 1200° C. 1225° C. 1250° C. 0.003 0.008 0.0070.007 0.006 0.006 0.005 0.011 0.010 0.010 0.009 0.008 0.008 0.015 0.0140.013 0.012 0.011 0.010 0.019 0.017 0.016 0.015 0.014 0.013 0.022 0.0210.019 0.018 0.016 0.015 0.026 0.024 0.022 0.020 0.019 0.018 0.029 0.0260.024 0.023 0.021 0.020 0.031 0.029 0.027 0.025 0.023 0.023 0.034 0.0320.029 0.027 0.025 0.025 0.037 0.034 0.032 0.029 0.027 0.028 0.040 0.0370.034 0.031 0.029 0.030 0.042 0.039 0.036 0.033 0.031

TABLE 1C SiCl4 Mole X at X at X at X at X at X at Fraction T = 1000° C.T = 1025° C. T = 1050° C. T = 1075° C. T = 1100° C. T = 1125° C. 0.040.087 0.080 0.073 0.067 0.061 0.056 0.07 0.127 0.116 0.107 0.098 0.0900.083 0.1 0.160 0.147 0.135 0.124 0.115 0.106 0.13 0.188 0.173 0.1600.147 0.136 0.126 0.16 0.213 0.197 0.182 0.168 0.155 0.144 0.19 0.2360.218 0.202 0.187 0.173 0.161 0.22 0.256 0.237 0.220 0.204 0.189 0.1760.25 0.275 0.255 0.237 0.220 0.205 0.191 0.28 0.292 0.271 0.252 0.2350.219 0.204 0.31 0.308 0.287 0.267 0.249 0.232 0.217 0.34 0.323 0.3010.281 0.262 0.245 0.229 0.37 0.337 0.315 0.294 0.275 0.257 0.240

TABLE 1D SiCl4 X at X at X at X at X at Mole T = T = T = T = T =Fraction 1150° C. 1175° C. 1200° C. 1225° C. 1250° C. 0.04 0.052 0.0480.044 0.041 0.038 0.07 0.077 0.071 0.066 0.061 0.057 0.1 0.098 0.0910.084 0.079 0.073 0.13 0.117 0.108 0.101 0.094 0.088 0.16 0.134 0.1240.116 0.108 0.101 0.19 0.150 0.139 0.130 0.121 0.113 0.22 0.164 0.1530.143 0.133 0.125 0.25 0.178 0.166 0.155 0.145 0.136 0.28 0.190 0.1780.166 0.156 0.146 0.31 0.202 0.189 0.177 0.166 0.156 0.34 0.214 0.2000.188 0.176 0.165 0.37 0.225 0.211 0.197 0.185 0.174

In one embodiment, the uniformity of doped chlorine concentration(uniformity of the doped chlorine profile) is improved by compensatingthe lower SiCl₄ concentration at the inner locations of the soot layer(due to the diffusional limitation as the SiCl₄ is exposed from outside)by having the inner locations of the soot layer at higher temperaturesduring the SiCl₄ exposure step, than the outer locations of the sootlayer. Since the preform is heated in a consolidation/sintering furnacefrom outside, the soot layer generally is heated from outside to inside,with the temperature at the outer location higher than the insidelocations. To achieve the doping conditions where the temperature at theinside locations of the soot layer (e.g., at the location of 1 mm wideinner most region or portion of this layer) are higher than thetemperature at the outer locations (e.g., higher than the temperaturesof the outer most 1 mm portion/region of this layer), the soot layer isinitially heated from outside at (furnace) temperatures above 1250° C.As the inside locations of the soot layer heats and reaches atemperature T_(inside) that is higher than T_(dop), wherein the dopingtemperature T_(dop) is chosen for a choice of soot layer density ρ andSiCl₄ mole fraction y_(SiCl4) such that the parameter X is larger than0.03, more preferably larger than 0.04 and even more preferably largerthan 0.05, the temperature of the furnace is lowered to T_(dop) and thesoot layer is exposed to a doping environment comprising of a gasmixture with SiCl₄ (i.e., SiCl₄ exposure step) having mole fractiony_(SiCl4). The difference between T_(inside) and T_(dop) is preferablyhigher than 25° C., more preferably higher than 35° C. and even morepreferably higher than 40° C. The difference between T_(inside) andT_(dop) is preferably smaller than 80° C., more preferably smaller than60° C. and even more preferably smaller than 50° C. PreferablyT_(inside) is less than 1250° C., more preferably less than 1225° C. andeven more preferably less than 1200° C.

In another embodiment, the mole fraction of the SiCl₄ is increasedduring the exposure of the soot layer to the gas stream comprising theSiCl₄ dopant. The SiCl₄ mole fraction in the gas stream at the end ofthe SiCl₄ exposure process is preferably at least 1.25 times the SiCl₄mole fraction at the beginning of the SiCl₄ exposure process, morepreferably at least 1.5 times the SiCl₄ mole fraction at the beginningof the SiCl₄ exposure process, and even more preferably at least 2 timesthe SiCl₄ mole fraction at the beginning of the SiCl₄ exposure process.In yet another embodiment, the uniformity of doped chlorineconcentration (uniformity of the doped chlorine profile) is improved bycompensating the lower SiCl₄ concentration at the inner locations of thesoot layer (due to the diffusional limitation as the SiCl₄ is exposedfrom outside) by having the inner locations of the soot layer attemperatures that are 25° C. to 80° C. higher than the dopingtemperature T_(dop) during the SiCl₄ exposure, with the SiCl₄ molefraction at the end of the SiCl₄ exposure process at least 1.5 timeshigher than the SiCl₄ mole fraction at the beginning of the SiCl₄exposure process during the SiCl₄ exposure of the soot layer to the gasstream comprising the SiCl₄ dopant. In these different embodiments, thesoot layer is preferably pre-dried by exposure to mixture of chlorineand helium gas (comprising chlorine between 1% and 5%) prior to exposureof the soot layer to the gas stream comprising the SiCl₄ dopant.

In yet another embodiment, the soot preform is initially treated withSiCl₄ containing gas at a temperature at which no appreciable silicachlorination reaction takes place. This initial treatment temperature ispreferably less than 900° C. such that SiCl₄ is able to successfullydiffuse to the inner regions of the soot layer. The temperature of thesoot layer is subsequently increased to temperatures above 1000° C. andthe SiCl₄ already diffused in then reacts with silica to chlorine dopeit.

The resultant chlorine concentration on the inside edge of theouter-cladding layer of the preform (and thus that of outer claddinglayer 3 of the optical fiber) is greater than 40% of the maximumchlorine concentration in the outer cladding layer, more preferablygreater than 50% of the maximum chlorine concentration in the outercladding layer, and even more preferably greater than 75% of the maximumchlorine concentration in the outer cladding layer. The uniformity ofthe profile helps with reducing excess stress in the core andinner-cladding, which helps in lowering fiber attenuation. Theuniformity of the profile also improves the optical performance of thefiber, including the bend performance of the optical fiber.

According to some embodiments, the step of exposing a silica basedpreform with at least one porous glass region to SiCl₄ at temperatureT_(dop) below 1250° C. also includes simultaneously exposing the silicabased preform with at least one porous glass region to carbon monoxide(CO) gas for at least part of the SiCl₄ exposure process. In someembodiments, for part of the SiCl₄ doping process, use of SiCl₄ incombination with CO is also disclosed that helps in effectivedehydration of the outer-cladding layer, as well as reducing thenon-bridging oxygen defect concentrations in the outer cladding layer.The concentration of CO used may be between 0 and 10000 ppm by volume,more preferably between 500 and 6000 ppm by volume and even morepreferably between 1000 and 4000 ppm by volume. The fiber made with sucha method exhibit attenuation at 1550 nm that is less than 0.185 dB/km at1550 and a bend loss for a 20 mm mandrel diameter that is less than 0.1dB/turn.

The invention is further illustrated using following examples.

Example 1

A soot layer was pre-heated from outside at a furnace temperature of1275° C. For part of the pre-heat step, the soot layer was treated witha mixture of chlorine and helium comprising 1-2% chlorine. When theinside of the soot-layer heats up to 1175° C., the temperature of thefurnace is dropped to 1125° C. The soot layer is subsequently treatedwith a gas stream with SiCl₄ mole fraction of 0.02 for 4.3 hrs. The sootlayer is then sintered to fully dense glass in a SiCl₄ free environmentat a temperature higher than 1450° C. The chlorine doped profile in thefully densified glass is shown in FIG. 5 (solid line).

Example 2

A soot layer was pre-heated from outside at a furnace temperature of1275° C. For part of the pre-heat step, the soot layer was treated witha mixture of chlorine and helium comprising 1-2% chlorine. When theinside of the soot-layer heats up to 1050° C., the temperature of thefurnace is dropped to 1125° C. The soot layer is subsequently treatedwith a gas stream with SiCl₄ mole fraction of 0.02 for 2.3 hrs. The sootlayer is then sintered to fully dense glass in a SiCl₄ free environmentat a temperature higher than 1450° C. The chlorine doped profile in thefully densified glass is shown in FIG. 5 (curve with small dashes).

Example 3

A soot layer was pre-heated from outside at a furnace temperature of1275° C. For part of the pre-heat step, the soot layer was treated witha mixture of chlorine and helium comprising 1-2% chlorine. When theinside of the soot-layer heats up to 1175° C., the temperature of thefurnace is dropped to 1125° C. The soot layer is subsequently treatedwith a gas stream comprising SiCl₄ for 2 hours with SiCl₄ mole fractionincreasing from 0.023 at the beginning of the SiCl₄ gas stream exposureto 0.04 at the end of the SiCl₄ gas stream exposure. The soot layer isthen sintered to fully dense glass in a SiCl₄ free environment at atemperature higher than 1450° C. The chlorine doped profile in the fullydensified glass is shown in FIG. 5 (curve with larger dashes).

Preferably the chlorine doped preform is sintered at the temperaturesabove 1400° C. to completely sinter the preform, for example such thatits glass density is at least 2.2 g/cm³.

Accordingly, in at least some of the embodiments a method of making anoptical fiber preform comprises the steps of:

(i) exposing a silica based preform with at least one porous glassregion having soot density of ρ to a gas mixture comprising SiCl₄ havingSiCl₄ mole fraction y_(SiCl4) of less than 0.03 at a doping temperatureT_(dop) such that parameter X is larger than 0.03 to form the chlorinetreated preform, wherein

$X = \frac{1}{1 + \left\lbrack {\left( \frac{\rho}{\rho_{s} - \rho} \right)\frac{0.21T_{dop}{{Exp}\left\lbrack {{- 5435.33}/T_{dop}} \right\rbrack}}{y_{{SiCl}\; 4}^{3/4}}} \right\rbrack}$and ρ_(s) is the density of the fully densified soot layer; and(ii) exposing the chlorine treated preform to temperatures above 1400°C. to completely sinter the preform to produce sintered optical fiberpreform with a chlorine doped region having a chlorine doped profile.

Preferably, X>0.03, more preferably X>0.04. Preferably the ratio of theconcentration of doped chlorine in the inside (e.g., a 1 mm wide innermost portion) of the Cl doped region to maximum concentration of dopedchlorine in the Cl doped region is >0.4, more preferably >0.5, and evenmore preferably X>0.75. In some embodiments the ratio of theconcentration of chlorine in the inside portion of the Cl doped region(e.g., the 1 mm wide inner most portion of the Cl doped region) toconcentration of chlorine in the outer portion of the Cl doped region(e.g., the 1 mm wide outer most portion of the Cl doped region))is >0.4. Preferably T_(dop) is less than 1225° C., more preferably lessthan 1200° C. and even more preferably less than 1175° C. Preferably,the mole fraction of SiCl₄ in the gas mixture (in carrier gas) y_(SiCl4)is larger than 0.005, more preferably larger than 0.01, and in someembodiments larger than 0.015.

In at least some embodiments, the difference between T_(inside) andT_(dop) is less than 80° C., in some embodiments less than 60° C. Insome embodiment wherein the soot layer that is being doped with Cl isexposed to carbon monoxide, wherein the concentration of carbon monoxideis between 0 and 10000 ppm by volume in the gas mixture. (T_(dop) isdoping temperature of the gas(es) directly surrounding (and in contactwith) the preform.)

According to some embodiment the method of making an optical fiberpreform comprises the steps of: (i) exposing a silica based preform withat least one porous glass region having soot density of ρ<0.8 gm/cm³ toa gas mixture comprising SiCl₄ having SiCl₄ mole fraction y_(SiCl4) ofless than 0.03 at a doping temperature T_(dop)<1225° C.; and (ii)exposing the chlorine treated preform to temperatures above 1400° C. tocompletely sinter the preform to produce sintered optical fiber preformwith a chlorine doped region having a chlorine doped profile. In theembodiments disclosed herein ρ<0.8 gm/cm³, preferably ρ<0.6 gm/cm³, forexample ρ<0.6 gm/cm³, or ρ<0.6 gm/cm³, and in some embodiments ρ<0.5gm/cm³.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method of making an optical fiber preformcomprising the steps of: (i) exposing a silica based preform with atleast one porous glass region of silica based soot with soot density ofρ at a doping temperature T_(dop) to a gas mixture comprising SiCl₄ andhaving SiCl₄ mole fraction y_(SiCl4) such that parameter X is largerthan 0.101 to form a chlorine (Cl) treated preform with Cl doped region,wherein$X = \frac{1}{1 + \left\lbrack {\left( \frac{\rho}{\rho_{s} - \rho} \right)\frac{0.21T_{dop}{{Exp}\left\lbrack {{- 5435.33}/T_{dop}} \right\rbrack}}{y_{{SiCl}\; 4}^{3/4}}} \right\rbrack}$ρ_(s) is the density of the fully densified soot layer and ρ<ρ_(s); and(ii) exposing the chlorine treated preform to temperatures above 1400°C. to completely sinter the chlorine treated preform and producing asintered optical fiber preform with a chlorine doped region having achlorine doped profile.
 2. The method of making an optical fiber preformaccording to claim 1, wherein said step of exposing the chlorine treatedpreform to temperatures above 1400° C. to completely sinter the chlorinetreated preform is performed in essentially SiCl₄ free environment. 3.The method of making an optical fiber preform according to claim 1,wherein chlorine doping profile of the Cl doped region is such that theratio of the concentration of doped chlorine in an inner portion of theCl doped region to maximum concentration of doped chlorine in the Cldoped region is ≧0.4.
 4. The method of making an optical fiber preformaccording to claim 1, wherein chlorine doping profile is such that theratio of the concentration of doped chlorine in the inner portion of theCl doped region to concentration of doped chlorine in the outer portionof the Cl doped region is ≧0.75.
 5. The method of making an opticalfiber preform according to claim 1, wherein T_(dop) is less than 1225°C.
 6. The method of making an optical fiber preform according to claim1, wherein T_(dop) is less than 1200° C.
 7. The method of making anoptical fiber preform according to claim 1, wherein the mole fraction ofSiCl₄ in the gas mixture y_(SiCl4) is larger than 0.005.
 8. The methodof making an optical fiber preform according to claim 1, wherein themole fraction of SiCl₄ in the gas mixture y_(SiCl4) is larger than0.015.
 9. A method of making an optical fiber preform according to claim1, wherein the parameter X is not less than 0.15.
 10. The method ofmaking an optical fiber preform according to claim 1, wherein theparameter X is not less than 0.2.
 11. The method of making an opticalfiber preform according to claim 1, wherein the parameter X is not lessthan 0.301.
 12. A method of making an optical fiber preform comprisingthe steps of: (i) making a glass preform comprising of a core region anda first cladding region; (ii) overcladding the glass preform with silicasoot overclad layer having silica soot with soot density ρ, therebyforming a silica soot overclad layer; (iii) exposing the silica sootoverclad layer at a doping temperature T_(dop) to a gas mixturecomprising SiCl₄ and having SiCl₄ mole fraction y_(SiCl4) such thatparameter X is larger than 0.101 to form a chlorine (Cl) treated preformwith Cl doped overclad region, wherein$X = \frac{1}{1 + \left\lbrack {\left( \frac{\rho}{\rho_{s} - \rho} \right)\frac{0.21T_{dop}{{Exp}\left\lbrack {{- 5435.33}/T_{dop}} \right\rbrack}}{y_{{SiCl}\; 4}^{3/4}}} \right\rbrack}$ρ_(s) is the density of the fully densified soot layer and ρ<ρ_(s); and(iv) exposing the chlorine treated preform to temperatures above 1400°C. to completely sinter the chlorine treated preform and producing asintered optical fiber preform with a chlorine doped overclad regionhaving a chlorine doped profile, wherein the chlorine doped overcladregion comprises chlorine in an amount greater than 2000 ppm by weight.